Minor technical point: There is no way you could carbon date anything to an age of 400 million years. The half-life of C-14 is too short to go back that far. There would be an essentially undetectable amount of C-14 left in anything that old. There are other radiometric dating methods capable of going back that far, however, so the main point is still valid.
Sorry, didn't see your correction re: the carbon dating thing above.